Asthma is a chronic inflammatory disease and is frequently associated with an allergic component. Asthma poses a major public health burden. Prevalence of this disease has increased greatly over the last four decades yet many questions regarding the basic mechanisms that lead to this disease remain unanswered. High-affinity, allergen-specific IgE antibodies are strongly implicated in the pathogenesis of allergic asthma, as are pathways involved in IgE production such as signaling by interleukin-4 (IL-4). A unique subset of CD4 T cells termed follicular helper T cells (Tfh cells) are present in the germinal center (GC) where they provide critical help in the production of high-affinity antibodies as well as isotype class switching, including to IgE, via production of IL-4. Tfh cells also secrete IL-21,a cytokine that represses switching to IgE, suggesting a complex relationship between Tfh cells, cytokine production, and isotype switching. This proposal seeks to test the hypothesis that Tfh cells regulate the production of IL-4 and IL-21 independently to coordinate the GC response and evoke IgE production. Three specific aims are proposed to interrogate this hypothesis. The first aim will investigate the relationship between Tfh cells and the humoral response in vivo by defining subpopulations of Tfh cells by their cytokine secretion on the basis of intracellular staining as well as using mutant mice expressing fluorescent reporter genes. The composition of these populations will be measured in relation to antibody titer and the GC response in several models of infection and immunization and across time and anatomical location. The second aim is to directly connect different subpopulations of cytokine-secreting Tfh cells with the GC B cells they are interacting with, and to utilize an adoptive transfer system to test the function of these individual populations. In the third aim, regulatory networks controlling cytokin production will be determined using advanced techniques in next-generation sequencing and computational biology. Altogether, this work will further clarify the mechanism by which Tfh cells control allergy- inducing IgE antibodies and in doing so will identify targets for disrupting this process. In addition, this application details the applicant's training plan including research mentorship, advanced coursework, training in new techniques, and development of skills in scientific professionalism, writing, and presentation of data. The development of new treatments for major diseases like asthma depends on projects like this one which translate clinical problems into basic research questions. The research and training outlined in this application will prepare the applicant to pursue a career in the conduct of clinically relevant research as an independent physician scientist.